1. Field of the Invention
The present invention relates to a radio-frequency power amplifier (RF power amplifier hereinafter), and more particularly, to a RF power amplifier with high output efficiency and a wide range of gain control.
2. Description of the Related Art
In general, RF power amplifiers are used in wireless communication systems to amplify and transmit signals. High efficiency and output power are the necessary requirements of RF power amplifiers. Furthermore, in order to be appropriate for various communicating distances at different occasions, the RF power amplifier must be able to provide a wide range of gain control such that the output power can be adaptively controlled according to the communicating distance, thereby reducing unnecessary output power consumption.
At present, the various methods for controlling RF power amplifiers applied in monolithic microwave integrated circuits (MMIC) include: (1) a power-synthesizing method, (2) a power-attenuating method, (3) the unbalanced bias cascode (UBIC) method, (4) a drain voltage controlling method, and (5) a gate voltage controlling method. The drawbacks of the above five methods are described as follows.
(1) With the power-synthesizing method, a plurality of power synthesizers (or power amplifiers) is combined to achieve output power control. However, giving the relatively high cost of the synthesizers, the number of the synthesizers always is restricted to reduce the cost, thereby reducing the possible options for the output power level. In addition, the control circuits are very complicated.
(2) With the power-attenuating method, power attenuators are disposed in the RF output circuits to directly attenuate the output power, thereby achieving output power control. However, it is very difficult to design attenuators with a wide range of power-attenuating capacity, low power consumption, and lower cost for MMIC applications.
(3) With the UBIC method, a common-source FET (field-effect transistor) and a common-gate FET are cascoded (connected in series) to serve as an amplifying stage, and a plurality of amplifying stages are cascoded to work as a power amplifier. By varying the gate voltages at the gate of the common-gate FETs, the drain currents of the common-source FETs are varied in response, thereby achieving output power control. However, the control circuits applied in the UBIC method are complicated and costly. In addition, the amplifiers are vulnerable to oscillation and instability, because impedance matching between amplifying stages is very difficult.
(4) With the drain-voltage control method, a source-control transistor is added to the source input of the RF power transistor, and the drain current of the RF power transistor can be adjusted by varying the gate voltage of the RF power transistor, thereby achieving output power control. However, the fabrication cost is very high.
(5) The gate-voltage control method will be described as follows, with reference to FIG. 1.
In the case of the gate-voltage control method, the RF power amplifier comprises an input-stage amplifier, an intermediate-stage (or driving-stage) amplifier, and a power-stage amplifier. The above three amplifiers are serially connected. The above three stage amplifiers consist of RF power transistors Q1, Q2, Q3 with bias circuits respectively. The bias sources Vg1, Vg2, Vg3 are used to bias the gates of the power transistors Q1, Q2, and Q3 to adjust the operating point of the power transistors Q1, Q2, and Q3 respectively. General speaking, the input-stage amplifier functions in class A or Class AB mode by adjusting the operating point of the RF power transistor Q1. The input-stage amplifier is used to first amplify the input signal RF.sub.in. The power-stage amplifier always functions in Class AB mode by adjusting the operating point of the RF power transistor Q3. By varying the gate biases of the transistors Q1, Q2, and Q3, the amplification ratios of the three amplifier stages can be adjusted, and therefore the RF power amplifier can amplify the input signal RF.sub.in to the expected power level.
The gate-voltage control method is simple, but there are still some drawbacks. Because the gate voltage Vgs of the RF power transistor only has a narrow voltage control range, a wide range of gain control and precise output power control are very difficult to achieve. Furthermore, the power-stage amplifier functions in class AB mode, and thus it is always in the turned-on state. Consequently, a great deal of power is dissipated by the power-stage amplifier. Furthermore, because the power-stage amplifier is always in the turned-on state, the noise generated between every amplifier stage is amplified and output.